The present invention relates generally to the control of overhead cranes, and more particularly, to a system and method for controlling the speed, direction, and braking of an electric overhead bridge traveling crane.
As shown in FIG. 6, a conventional overhead crane 200 typically includes a horizontal section known as a “bridge” 204 that moves the suspended load across a horizontal span 202 over the work area. Movement of the bridge 204 is provided by a reversible, variable-speed electric motor coupled by conventional electromechanical transmission components to the bridge 204. The speed and direction of the motor, in turn, is powered and controlled by a variable frequency motor drive (VFD).
Conventionally, crane bridges are controlled by an operator using manual controls. A master switch (FIG. 2) is used by the crane operator to command motion, direction, torque/speed, and acceleration. A foot-operated hydraulic braking system (FIG. 3) is employed for slowing or stopping movement of the crane independently of the motor control. The hydraulic brake is operated in a fashion similar to that of an automobile where a master cylinder is operated by a foot pedal which in turn transmits hydraulic pressure to engage the brake or brakes. The hydraulic pressure and braking torque developed by the master cylinder is proportional to the amount of force applied to the foot pedal. Because braking on these conventional overhead bridge cranes is controlled by the operator, the crane is essentially coasting when the crane operator moves the master switch to the neutral position.
Conventional crane control systems also use a braking system that includes a ramped deceleration time associated with the variable frequency drive. Most of the traditional crane controls use techniques and systems that are not efficient and also cause abnormally large amounts of destructive forces on the motor and drive systems of these conventional overhead cranes.
For example, conventional crane controls use “plugging” as a technique for controlling the movement of the crane. Plugging is a term that has carried over from traditional contactor controls where a motor is connected directly to the line through the use of reversing contactors. Plugging is defined as a control function that provides braking by reversing the motor line voltage polarity, or phase sequence, so that the motor develops a counter torque that exerts retarding force. This method of slowing or stopping the crane is inherently detrimental to the motor and controls as it subjects them to several times the amount of nominal current. See Charts 1 and 2 below for an example of this detrimental effect. Chart 1 shows a representation of conventional voltage and current spikes associated with supplying a voltage and current with the same polarity as the voltage and current present within motor rotating in a given direction. Chart 2 shows a representation of conventional current and voltage spikes associated with supplying a reversed plurality voltage and current to a motor traveling in a given direction. This is also known as reverse plugging.
CHART 1Reversing contactor (Same Direction)Motor - 3 Phase Induction MotorChart InformationRated Voltage -Rated Current -A1 -Forward contactor -460 Vac3.0 AmpsLogic 0 or 1Open/ClosedRated RPM -RatedA2 -Reverse contactor -1750Horsepower - 2Logic 0 or 1Open/ClosedDesign -Frame -CH1 -Motor Voltage -NEMA B145 TCZ100 Vac/DivisionT2, T3TENVContinuousCH2 -Motor Current -Duty10 mv/AmpT3
CHART 2Reversing contactor(Opposite Direction - Reverse Plugging)Motor - 3 Phase Induction MotorChart InformationRated Voltage -Rated Current -A1 -Forward contactor -460 Vac3.0 AmpsLogic 0 or 1Open/ClosedRated RPM -RatedA2 -Reverse contactor -1750Horsepower - 2Logic 0 or 1Open/ClosedDesign -Frame -CH1 -Motor Voltage -NEMA B145 TCZ100 Vac/DivisionT2, T3TENVContinuousCh2 -Motor Current -Duty10 mv/AmpT3
Also, conventional controls do not facilitate adjustments to the speed to the crane. In fact, conventional crane controls lack efficient and non-destructive speed adjustment even when the user desires a speed adjustment in the same direction in which the crane is traveling. For example, in a traditional reversing contactor control system, the contactors are de-energized and the circuit to the motor is opened when the directional switch is in the neutral position. In this position, the crane will be coasting at some speed. During this coasting period, the residual voltage and current in the motor will decay as the rotor demagnetizes. The amount of time for the voltage and current to decay is dependent on several variables: motor horsepower, motor type, load, temperature, etc. Usually the voltage and current have completely decayed prior to a desired speed change by a user of the crane. Also, after some period of time, the speed of the crane, and hence the motor speed, can decrease from friction or the application of a brake.
When a user decides to alter the velocity of a crane using a conventional control system, the user activates the conventional control system from a neutral position. In turn, line voltage and frequency are applied to the coasting motor. When the motor is re-energized with the desired voltage and frequency corresponding to a desired speed, the rotation of, and hence the frequency and voltage within, the motor is not synchronous with that of the line voltage and frequency being applied by the user. In other words, the residual voltage and frequency in the motor is not equal to the desired voltage and frequency being applied by the user.
The motor follows the direction and frequency of the line voltage being applied to it. As such, the applied frequency will either accelerate or decelerate the motor, and the crane, to follow the commanded speed and direction from the applied frequency. This can cause sizeable current transients, vibrations, and significant wear to the motor, controls and machinery over time. The effects of the transients, vibrations, and significant wear can be more than tripled when the controls are reversed plugged. Charts 1 and 2 above illustrate this detrimental effect on a motor with an inertia load, the motor controlled by a set of reversing contactors.
Thus, there is a need an overhead crane bridge control system that effectively and efficiently controls the velocity and direction of the crane without undo wear to the motor, controls, and machinery of the crane.